编程题：Calculate ball volume

Input the radius to calculate the volume of the ball. The ball volume formula is:$$\frac{4}{3}\pi{r^{3}}$$
please use the pi constant of the math library as $$\pi$$. (math.pi)

### Input Specification:

Enter a radius, which can be a floating-point number.

### Output Specification:

Output the volume of the ball, and keep 3 decimal places for the result.

### Sample Input #1:

in
2


### Sample Output #1:

out
v=33.510

### Sample Input #2:

in
3.33


### Sample Output #2:

out
v=154.675










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